Most conventional Global Positioning System (GPS) receivers utilize serial correlators in order to acquire, track, and demodulate signals transmitted from Navstar satellites. Each transmitted GPS signal is a direct sequence spread spectrum signal. The signal available for commercial use is that associated with Standard Positioning Service (SPS) and utilizes a direct sequence biphase spreading signal with a 1.023 Mchip per second spread rate placed upon a carrier at 1575.42 MHz. The Pseudo-random Noise (PN) sequence length is 1023 chips, corresponding to 1 msec time period. Each satellite transmits a different PN code (Gold code) which allows the signals to be simultaneously transmitted from several satellites and to be simultaneously received by a receiver, with little interference from one another. In addition, data superimposed on each signal is 50 baud binary phase shift keyed (BPSK) data with bit boundaries aligned with the beginning of a PN frame; 20 PN frames occur over 1 data bit period (20 msec).
A primary goal of a GPS receiver is to determine the time-of-arrival of the PN codes. This is accomplished by comparing (for each received signal) a locally generated PN reference against the received signal and "sliding" the local reference in time until it is time-aligned with the received signal. The two signals are compared with one another by a multiplication and integration process known as a correlation process. When the two signals are time aligned a large output results. Typical serial correlators used in standard GPS receivers compare the local and received signals one time offset at a given time. If such a comparison is done every half-chip interval, there are 2046 comparisons (or tests) required to completely search over one PN epoch. Such a search must be done for several of the satellites in view. In addition, errors in received signal frequency often require additional searches to be made for various hypotheses of signal frequency. The time to perform this search may be very lengthy, especially under low input signal-to-noise ratio situations. Conventional GPS receivers utilize a multiplicity of such correlators operating in parallel to speed up the acquisition process. Nevertheless, the search and acquisition process is very time consuming, especially in low received signal-to-noise situations.
It is therefore desirable to provide a hardware architecture that improves the acquisition speed and sensitivity of current conventional GPS receivers. Such an architecture would allow the receiver to operate at a very low input signal-to-noise ratio. It is further desirable to integrate a method for tracking such signals, following the acquisition procedure, in which a commonality of hardware is used for both the acquisition and tracking of received GPS signals.